As the field of plant bioengineering develops and more genes become accessible, a greater need exists for transforming with multiple genes. These multiple exogenous genes typically need to be controlled by separate regulatory sequences. Some genes should be regulated constitutively whereas other genes should be expressed at certain developmental stages or location in the transgenic organism. Accordingly, a variety of regulatory sequences having diverse effects is needed.
In addition, undesirable biochemical interactions can result from using the same regulatory sequence to control more than one gene. For example, transformation with multiple copies of a regulatory element may cause homologous recombination between two or more expression systems, formation of hairpin loops caused from two copies of the same promoter or enhancer in opposite orientation in close proximity, competition between identical expression systems for binding to common promoter-specific regulatory factors, and inappropriate expression levels of an exogenous gene due to trans effects of a second promoter or enhancer.
In view of these considerations, a goal in this field has been the detection and characterization of new regulatory sequences for transgenic control of DNA constructs.
Isolation and characterization of constitutive promoters and terminators that can serve as regulatory elements for expression of isolated nucleotide sequences of interest in a constitutive manner are needed for improving traits in plants.